The "multilayer" technology is of increasing importance in the field of ceramic components for electronics. It is widely employed for producing passive components such as capacitors, enclosing boxes for integrated circuits and interconnection substrates of hybrid circuits. This technology may be briefly divided up into three steps:
pouring from a barbotine and drying the ceramic in the form of bands whose thickness is usually between 20 and 200 microns;
making up the component in the raw state essentially by a metallization and a stacking of the ceramic layers: the metallization of pre-cut ceramic bands is achieved by deposition, in particular by serigraphy, of an ink whose principal constituent is a metallic powder, so as to form the electrodes of the capacitors, the electrical paths of the enclosing boxes and of the interconnection substrates (in the latter two cases, there must be included the operations for perforating some of the ceramic bands and the metallization of the "vias" thus obtained); the stacking of the metallized layers, followed by a hot pressing (about 150.degree. C.), results in a component in the raw and dry state, with a structure similar to structure it will have after firing;
cofiring the raw components.
The cofiring, sometimes also designated by the term "cosintering", is a complex heat treatment operation having a plurality of successive stages in the course of which it is attempted to achieve:
the elimination of the organic products contained in the "raw" components;
the cosintering proper of the ceramic and metallic layers.
The elimination of the organic products, which must be as complete as possible, is effected in a progressive manner during the rise in temperature. The cosintering is achieved by providing a temperature plateau at a value which is a function of the materials to be treated and is usually between about 800.degree. C. and about 1800.degree. C. The cofiring is achieved at low temperature (from about 800.degree. C. to about 1300.degree. C.) for ceramic materials such as cordierite and metallic materials such as copper and nickel, whereas it is achieved at high temperature (from about 1400.degree. C. to about 1800.degree. C.) for refractory materials such as tungsten, molybdenum, alumina, aluminium nitride. During the cofiring, a densification of the component occurs with linear shrinkages which may attain 20% to 30%. According to the cases, large ceramic-metal interface reactions may also occur. The treatment is terminated by a cooling down to ambient temperature.
This well-known "multilayer" technology is in particular described in respect of refractory materials by B. SCHWARTZ and D. L. WILCOX in the communication entitled "Laminated Ceramics" which appeared in "Proc. Electron Comp. Conf.", WASHINGTON, D.C., 1967.
During this heat treatment operation, the function of the atmosphere is essential. In the course of the first stages of the treatment, the atmosphere must promote and in any case permit the complete elimination of the organic products contained in the raw materials. In the subsequent stages of the treatment, the atmosphere permits controlling the size of the grains, the densification of the component, and acting on the ceramic-metal interface reactions. The atmosphere is also chosen in such manner as to avoid oxidizing certain of the materials employed, principally for the metals, tungsten, molybdenum, copper, and for the ceramics, "non-oxides" such as aluminium nitride. In certain cases, the atmosphere also permits regenerating at higher temperature the metals which would have undergone a partial oxidation when eliminating the binders. These functions of the atmosphere are achieved by regulating its oxydo-reduction potential in the course of the treatment. A simple means of effecting this regulation consists in employing a more-or-less large additional supply of water vapour in an atmosphere constituted by hydrogen and an inert gas such as nitrogen, or if desired, in an atmosphere otherwise constituted solely by hydrogen, or even, on the contrary, in an atmosphere otherwise constituted solely by an inert gas.
The treatment atmospheres which are employed in these cofiring operations must have well-determined water vapour contents since these water vapour contents intervene both as such and in relation in a ratio with the hydrogen content so as to impart to the gas an oxidizing effect, a reducing effect, a neutral effect. The obtaining of moist atmospheres for the cofiring of ceramic-metal multilayers is at the present time achieved by a simple bubbling of the gas in a container containing water. However, this manner of proceeding presents the drawbacks of imposing a dew point roughly equal to the ambient temperature, and fluctuations of the dew point by variation of the ambient temperature and the water level in the bubbler. In order to obtain higher dew points at ambient temperature it is then necessary to heat the water of the bubbler and, if, on the contrary, it is desired to obtain lower dew points at ambient temperature, it is necessary to cool the water of the bubbler or dilute the gas saturated with water with a dry gas. Furthermore, in order to obtain a well-determined dew point, it is at the present time not only necessary to regulate the temperature of the water of the bubbler, and therefore to employ heat insulated baths with thermostats, but also necessary to regulate the water level, and therefore to employ a level probe coupled with an automatic filling device. An arrangement of this type has been found to be costly and, in addition, notwithstanding all these improvements, various drawbacks still subsist:
on one hand, the value of the dew point produced is a function of the geometry of the bubbler (in particular of the water/gas exchange surface), whence the obligation to determine by calibration the operational conditions of each bubbler (heating and dilution) so as to obtain the desired dew points; PA1 on the other hand, in the case where it is desired to change the dew point to a higher dew point, this change, obtained by a heating of the bath, presents the drawback of having a long response time; PA1 lastly, in the case where it is desired to change to a lower dew point, the cooling of the bath presents the same thermal inertia drawback as above, whereas the dilution presents the drawback of a change in the total flow when passing from one dew point to the other.
These drawbacks may certainly be avoided by a quasi-infinite extension of the water/gas exchange surface by passing, for example, through a large volume of porous material as described in the article entitled "Advanced gas Moisturizing System for Ceramic processing applications" by F. W. GIACOBBE, which appeared in "American Ceramic Society Bulletin", Volume 66, No. 10, 1987 and by conservation of the total flow by readjustment of the flows of moisturized gas and dry gas.
However, such a moisturizing system is still hardly flexible. Its construction is very costly and the problem of the thermal inertia, in the case where it is desired to change the temperature of the bath, is still not solved.